Three port transceiver

ABSTRACT

An optical coherent transceiver comprising a polarization and phase-diversity coherent receiver and a polarization and phase-diversity modulator on the same substrate interfaced by three grating couplers, on grating coupler coupling in a signal, one grating coupler coupling in a laser signal, and a third grating coupler coupling out a modulated signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation, and claims the benefit under 35U.S.C. §120, of U.S. patent application Ser. No. 13/733,108, filed onJan. 2, 2013, and entitled “THREE PORT TRANSCEIVER,” which applicationis incorporated herein by reference in its entirety. U.S. patentapplication Ser. No. 13/733,108 claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/582,387, filed Jan. 1, 2012 and entitled“SILICON COHERENT TRANSCEIVER WITH INTEGRATED LASER,” and U.S.Provisional Patent Application Ser. No. 61/638,651, filed Apr. 26, 2012,and entitled “THREE PORT TRANSCEIVER,” and U.S. Provisional PatentApplication 61/638,656, filed Apr. 26, 2012, and entitled “THREE PORTTRANSCEIVER,” all of which are incorporated by reference in theirentireties.

TECHNICAL FIELD

This disclosure relates generally to the field of telecommunications andin particular to a monolithic, phase- and polarization-diversitycoherent transceiver photonic integrated circuit.

BACKGROUND

Contemporary optical communications systems oftentimes employ opticaltransceivers. Given their importance to the communications arts,techniques, methods and apparatus that facilitate their operation orefficiency would be a welcome addition to the art.

SUMMARY

An advance in the art is made according to an aspect of the presentdisclosure directed to techniques, methods, apparatus and systemspertaining to monolithic, phase-and polarization-diversity coherenttransceiver photonic integrated circuits.

Viewed from a first aspect, the present disclosure is directed to anoptical coherent transceiver comprising a polarization andphase-diversity coherent receiver and a polarization and phase-diversitymodulator on the same substrate interfaced by three grating couplers,one grating coupler coupling in a modulated signal, one grating couplercoupling in a continuous-wave (cw) signal, and a third grating couplercoupling out a modulated signal.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present disclosure may be realizedby reference to the accompanying drawings in which:

FIG. 1 shows coherent link according to the Prior Art;

FIG. 2 shows a schematic top-view of an exemplary 3-port coherenttransceiver photonic integrated circuit (PIC) according to an accordingto an aspect of the present disclosure;

FIG. 3 shows a schematic side view of fiber assembly according an aspectof the present disclosure;

FIG. 4 shows a schematic top view of a 3-port coherent transceiver PICusing a tilted fiber array according to an aspect of the presentdisclosure;

FIG. 5 shows a schematic top view of a 3-port coherent transceiver PICusing a tilted fiber array according to another aspect of the presentdisclosure;

FIG. 6 shows a schematic top view of a 3-port coherent transceiver PICusing a tilted fiber array according to yet another aspect of thepresent disclosure;

FIG. 7 shows a schematic top view of a 3-port coherent transceiver PICusing a tilted fiber array in which modulators are arranged along longedge of PIC according to an aspect of the present disclosure;

FIG. 8 shows a schematic top view of 3-port coherent transceiver PICusing a tilted fiber array with modulators arranged along a side facetaccording to an aspect of the present disclosure;

FIG. 9 shows a schematic top view of 3-port coherent transceiver PICusing tilted fiber array with variable optical attenuators (VOAs) andhigh-speed photodiodes (PDs) according to an aspect of the presentdisclosure;

FIG. 10 shows a schematic of a 2-D grating coupler using a tilted fiberaccording to an aspect of the present disclosure;

FIG. 11 shows a schematic top view of 3-port coherent transceiver PICusing tilted fiber array having an interferometer connected totransmitter grating coupler to orthogonalize the polarizations accordingto an aspect of the present disclosure;

FIG. 12 shows a schematic top view of 3-port coherent transceiver PICusing a non-tilted fiber array according to a aspect of the presentdisclosure;

FIG. 13 shows a schematic top view of 3-port coherent transceiver PICusing a non-tilted fiber array according to another aspect of thepresent disclosure;

FIG. 14 shows a schematic top view of a 3-port coherent transceiver PICusing a non-tilted fiber array according to yet another aspect of thepresent disclosure;

FIG. 15 shows a schematic top view of a 3-port coherent transceiver PICusing a non-tilted fiber array according to yet another aspect of thepresent disclosure;

FIGS. 16( a) and 16(b) show a schematic top view of a 3-port coherenttransceiver PIC using a non-tilted fiber array according to yet anotheraspect of the present disclosure;

FIG. 17 shows a schematic of a PIC and an erbium-doped fiber amplifier(EDFA) according to an aspect of the present disclosure;

FIG. 18 shows a schematic top view of a 3-port coherent transceiver PICusing a non-tilted fiber array according to yet another aspect of thepresent disclosure wherein all grating couplers are 2-D couplers andtrenches are employed in the substrate; and

FIG. 19 shows a schematic top view of a 3-port coherent transceiver PICaccording to yet another aspect of the present disclosure wherein alaser extended cavity is also fabricated on the same substrate coupledto a separate optical gain chip including a test port.

DETAILED DESCRIPTION

The following merely illustrates the principles of the disclosure. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosure and are includedwithin its spirit and scope.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes toaid the reader in understanding the principles of the disclosure and theconcepts contributed by the inventor(s) to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, arcintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently-known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the invention.

In addition, it will be appreciated by those skilled in art that anyflow charts, flow diagrams, state transition diagrams, pseudocode, andthe like represent various processes which may be substantiallyrepresented in computer readable medium and so executed by a computer orprocessor, whether or not such computer or processor is explicitlyshown.

In the claims hereof any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementswhich performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means which can provide those functionalities as equivalentas those shown herein. Finally, and unless otherwise explicitlyspecified herein, the drawings are not drawn to scale.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the disclosure.

By way of some additional background, we note that generally, opticalpolarization- and phase-diversity coherent transmission systems areconstructed using a receiver module having two optical input/output(I/O) ports, a modulator module having two optical I/O ports, and twolaser modules each having one I/O port. Such systems arc physicallylarge, expensive and include many optical interconnects as shown inFIG. 1. As generally shown in that FIG. 1, light emitted from a laser ismodulated by a number of data streams through the effect of a modulatorand subsequently applied to an optical fiber span after output from apolarization beam splitter. This modulated light is received by apolarization beam splitter which splits the received light intoX-polarization (X-pol) and Y-polarization (Y-pol) components. TheseX-pol and Y-pol components are applied to a respective 90° hybrid alongwith a local oscillator signal wherein it is subsequently detected,digitized and processed resulting in output data that corresponds to themodulator data applied by the modulator. Coherent systems such as thosedepicted in FIG. 1 are well known in the art.

Turning now to FIG. 2, there is shown an exemplary 3-port PIC accordingto an aspect of the present disclosure. As will become readilyappreciated by those skilled in the art, the 3-port PIC depicted in FIG.2 is constructed as a monolithic photonic integrated circuit havingthree optical I/O ports, and a separate laser module thereby reducingthe size, cost, and number of optical interconnections as compared tothe prior art. Advantageously, the exemplary 3-port PIC employs thesame, single laser for both transmitter and local oscillator. Thisallows the requirement for only 3 optical I/O ports. In the prior art,one would have one receiver PIC, needing 2 optical I/O ports and onemodulator PIC, needing 2 optical I/O ports, for a total of 4 optical I/Oports. Reducing the number of optical I/O ports reduces the cost andfootprint of the transceiver. Since the laser is housed in a separatemodule, a device according to the present disclosure does not requirenormal temperature control mechanisms. Further features and operationalcharacteristics will become apparent in the following discussion.

As depicted in this FIG. 2 and throughout the remaining drawing figures,the following nomenclature is used. PD=photodiode; TOPS=thermoptic phaseshifter; VOA=variable optical attenuator; TPAD=two-photon absorptiondiode; GC=grating coupler; R=receive signal in port; L=laser in port;and T=transmit signal out port.

With continued reference to FIG. 2, light from a cw laser is applied toport “L” where it is optically conducted to both a receiver portion anda modulator portion of the PIC. Generally, the L port is a 1-D gratingcoupler which operates as an efficient fiber coupler. Advantageously,when a focusing 1D grating coupler is employed as the L port, a shortoutput taper is possible. Ports “R” and “T” are 2-D grating couplers,which also serve as fiber couplers and polarization splitters androtators.

Once received by the L port, laser light is directed to both thereceiver and modulator portions of the PIC. Generally, the laser lightso received is split in a 50/50 manner, although other split ratio(s)are possible and contemplated.

An optical signal is received by the R port grating coupler and isdirected to two optical 90° hybrids and integrated photodiodes (PDs). Asmay be appreciated, the photodiodes may be advantageously constructed asGermanium on Silicon (Ge-on-Si) structures.

As may be observed from FIG. 2, modulator outputs are opticallyconnected to the T port grating coupler.

Advantageously, there are no waveguide crossings according to thepresent disclosure and therefore a low crosstalk and low insertion lossdevice is achieved. A further aspect of the present disclosure is thatthe receiver and modulator(s) are positioned at opposite ends of theoverall PIC, thereby minimizing any electrical and/or optical crosstalkor other interference during operation.

Received, Transmitted and Laser light is preferably coupled in-to andout-of the PIC using a 3-fiber array having high-index-contrast fibersuch that it may be bent to a small radius. A small bend radius isadvantageous as it permits a thin, overall profile while permitting thefibers to attach nearly perpendicular to the PIC surface.

FIG. 3 shows a schematic drawing of the fiber assembly according to anaspect of the present disclosure. As depicted in that FIG. 3, one mayobserve from the side view that the fiber assembly is shown as “tilted”by an amount of 8° from the perpendicular with respect to the PICsurface. Such tilting advantageously avoids strong Bragg backreflection(s) from the grating coupler(s) as light is directed fromwaveguide to a grating coupler.

With continued reference to FIG. 2, it may be observed that anintegrated received optical power monitor is in optical communicationwith the R port shown at the far left of the PIC depicted in FIG. 2. Asdepicted, the integrated received optical power monitor comprises aphotodiode that receives a portion from each of the receivedpolarization components. Advantageously, the same, single receivedoptical power monitor photodiodc receives the portions at opposite endsof the photodiode. As may be appreciated, such a structure may alsoinclude an integrated tunable filter—such as a ring resonator filterdepicted in FIG. 4. Such a filter may be tuned across a broad spectrumand thereby perform as in integrated spectrum analyzer.

As further depicted in FIG. 2 a pair of variable optical attenuators(VOAs) are positioned at the receiver R input. Such VOAs may be p-njunctions with carrier injection on a silicon waveguide. With suchdevices, as more carriers are injected the loss increases due tofree-carrier absorption.

The VOAs may used to advantageously increase the dynamic range of thereceiver. More specifically, when received signal power is high, theVOAs may be used to reduce the amount of signal reaching thephotodiodes. Consequently, this relaxes the dynamic range requirement ontransimpedance amplifiers (TIAs).

As may be further observed in FIG. 2, VOAs may be included on thetransmit output, positioned between the modulators and the T port. Thesealso may be p-n junction devices with carrier injection in the siliconwaveguide. Furthermore, taps and photodiodes (not specifically shown)may be positioned after the VOAs for providing a feedback signal to theVOAs. Alternatively, two-photon-absorption (TPA) diodes such as thoseshown may be employed. The VOA so positioned in the output mayadvantageously reduce transmit polarization dependent loss (PDL) as wellas control transmit power.

As may be appreciated, an inline TPA diode generally is areversed-biased p-n junction on the silicon waveguide. It produces aphotocurrent proportional to the square of the optical power passingthrough the waveguide. This diode serves a dual role of photo detectingand rectifying, which is needed to measure the RF power such that it maybe affected by a control loop that controls the modulator phase betweenthe I and Q components.

An alternative to using a TPA diode is to use a tap and a conventionalphotodiodc. Such an arrangement is shown schematically in FIG. 5. Inthis case shown, the PD photocurrent may be rectified to providefeedback for the 1-Q phase control.

Continuing with our general discussion with particular reference to FIG.2, the modulators shown comprise nested Mach-Zhendermodulators—sometimes referred to as “I-Q” modulators. Each of the twophase modulators comprising the nested unit Mach-Zehnder modulators arepreferably driven push-pull, and may be advantageously fabricated asdepletion modulators in silicon. Shown further in the figure, themodulators may include thermooptic phase shifters (TOPS). As may beappreciated, such TOPS are integrated heaters that locally heat awaveguide portion, changing its refractive index and thereby used toadjust the relative optical phase in the interferometers.

As described herein to this point, the L port depicted in FIGS. 2, 4,and 5 is a 1-D grating coupler. This is due to the fact that the inputlaser exhibits a single polarization, and a 1-D grating couplertypically exhibits a lower insertion loss than a 2-D grating coupler.Notwithstanding this noted advantage, those skilled in the art willappreciate that a 2-D grating coupler may be used instead, as depictedin FIG. 6.

As may be appreciated, alternative configurations using the same orsimilar R, L, and T configuration of grating couplers are possible andcontemplated, whereby the receiver PDs and transmitter modulators arearranged at different locations on the substrate chip—depending upon howthe electrical connections are made. For example, if it was desired tospread out the electrical connections—along one or more edges of thePIC, for example, the configuration shown in FIG. 7 may be employed. Inthis FIG. 7, the fiber array is tilted toward the left—in the drawingfigure—and the reader is directed to modulators located such that theyare spread out along the long edge of the PIC for easier electricalaccess.

Alternative configurations, such as the one depicted in FIG. 8 result inequal path lengths from the T grating coupler to the two I-Q modulators.In such configurations, rather than being tilted in the direction alongthe line made by the three grating couplers, the fiber array is nowtilted orthogonal to that line.

Advantageously, one can include the VOAs in the Rx prior to the PDs suchthat the common mode rejection ratio (CMRR) may be tuned. Such anarrangement is shown schematically in FIG. 9. The arrangement shown inFIG. 9 has the fiber array tilted toward the left in the figure andpermits the adjustment of the CMRR for the signal or the LO—but not bothsimultaneously. It is noted that it may be more likely that one wouldtune it for the signal as many channels could be simultaneously input tothe receiver but only one is detected. Of additional advantage, when theVOAs are positioned prior to the PDs as shown, it eliminates the needfor VOAs after the grating couplers—since these former VOAs can alsoincrease dynamic range.

All of the configurations according to the present disclosure anddepicted so far employ a tilted fiber array. As noted previously, thetilt to the fiber is used to avoid a strong second-order reflection oflight as it is conveyed from waveguide to fiber. If the fiber isperfectly vertical, then light reflected from the grating grooves willbe in phase. Since the grating is a second order grating (the period isa wavelength rather than a half wavelength—like a true Bragg grating),the reflection will be small, but not negligible and typically on theorder of 25%.

FIG. 10 shows a schematic of a 2-D grating coupler using tilted fiberaccording to an aspect of the present disclosure. The grating couplerdivides incoming light into two polarizations—X and Y—as depicted inFIG. 10. If the fiber is tilted, X and Y are not truly orthogonal.Instead, what is orthogonal is the diagonal basis set, S and P. Becausethe fiber is tilted along the plane that S is in, it experiences adifferent coupling efficiency. This is due to the fact that thepolarization excites odd modes in the grating, and S excites even modes.If this coupling efficiency is represented by η, (which varies withwavelength), then the following relationships hold:

X+Y=Sη; and

X−Y=P;

where X and Y are not orthogonal if is not equal to 1. One way to make ηequal to 1 is to modify the grating hole shapes such that the even andodd modes have the same effective index. However, this requires higherresolution lithography than is normally required for silicon photonicsand also may be difficult to reproduce consistently.

Another way, according to the present disclosure, is to use aninterferometer after the grating coupler as shown in FIG. 11. Where—asin FIG. 11—an interferometer is connected to the transmitter gratingcoupler the polarizations are made orthogonal. The interferometerinterferes the X and Y polarizations such as to recreate the S and Ppolarizations. Then on-chip VOAs can be used to control thepolarization-dependent loss (PDL).

Additionally, one could also place this interferometer (not specificallyshown in FIG. 11) on the receiver grating coupler. This position is ofless importance as the digital signal processor (DSP) in the receivercould perform the orthogonalization. It is more important for thetransmitter to launch orthogonal polarizations because of noise loadingdue to optical amplifiers in the transmission line. If a polarizationcomponent is weaker it will have a lower effective opticalsignal-to-noise ration. If a polarization component is weaker after thenoise loading, such as in the receiver, it will not have a reducedoptical signal-to-noise ratio.

Yet still another way to avoid this orthogonalization issue completelyis to not tilt the fibers at all. in such a case—and according to yetanother aspect of the present disclosure—we may reduce the backreflection significantly by employing a type of antireflection (AR)“coating” that comprises a short Bragg grating in the waveguide at thepoint where it connects to the grating coupler, appropriately phased.This is similar to etching a slot to cancel the second-order Braggreflection, but uses slots that are a quarter of a wavelength wide sothat they do not cause any upward scattering.

As may be appreciated, one advantage to having a vertical fiber is thatthe coupling efficiency is higher and the coupling bandwidth is larger.One disadvantage however, is that the light now emanates from both sidesof the grating coupler. To overcome this, we can either couple the twolightwaves in 2×1 couplers, as shown schematically in FIG. 12, or usethe splitting as 50/50 couplers as shown schematically in FIG. 13 andFIG. 14.

In the configuration depicted in FIG. 14, the R grating is splitting toboth the power monitors and the 90-degree hybrids. This results inapproximately 3 dB extra loss for the receiver. To reduce this loss, onecan apodize the R grating such that it sends more light to the receiverthan the power monitors. Similarly, with all of these configurations,the 1-D L grating may be apodized such that it sends a larger fractionof the laser power to the receiver or modulator, as desired.

FIG. 15 and FIGS. 16 a and b depict further arrangements according tothe present disclosure. In FIGS. 15 and 16 a, the 90-degree hybrids areconstructed from 1×2 and 2×2 couplers. In FIG. 16 b, the 90-degreehybrids are 2×4 couplers There are TOPS in the 2-D grating couplercombining paths for the R and T ports to control the relative phasebetween the two combining paths. These TOPS, in conjunction with theinterferometer formed by the grating coupler and combining paths, mayalso be used as VOAs when the combining paths are not perfectly inphase.

As may be appreciated, because a monolithic PIC such as those shown anddescribed have trade-offs as compared with discrete components, theinsertion loss is typically higher then with discrete components. Tomake up for this loss, we may advantageously position an Er-doped fiberamplifier on the transmitter output. As may be appreciated, thisamplifier may be pumped by an uncooled 980 or 1480 nm pump laser, whichtypically consumes less than 1 W of power. The optical amplifier may bepositioned within the same module as the PIC package. Such aconfiguration is shown schematically in FIG. 17.

FIG. 18 shows in schematic form yet another alternative configurationaccording to the present disclosure. In the configuration of FIG. 18,the L grating coupler is a non-tilted 2-D grating coupler. All fouroutputs are used as effective power splitters of the laser. This permitsthe L grating coupler to be exactly the same design as the R and Tgrating couplers, thereby minimizing design and fabrication issues.There are trenches etched between the grating couplers. This preventsscattered light from crossing directly between ports, since the L portwill typically exhibit a much higher power than the R and T ports.

Finally, FIG. 19, shows in schematic form an integrated receiver andmodulator on a single Si chip along with an integrated extended cavityfor a narrow line width tunable laser in the Si. The optical gain forthe laser comes from an InP chip that includes only a semiconductoroptical amplifier and therefore can be low cost and made by high yieldprocesses. The InP chip is attached to the Si chip preferably with anindex-matching adhesive.

As depicted in FIG. 19, one end of the cavity comprises HR-coated faceton the MP chip and the other end comprises ring resonators in the Si.The two ring resonators have slightly different free-spectral ranges, sothat they can be Vernier tuned to tune the wavelength over a widerrange. Light on resonance couples to the perpendicular waveguide andcouples back to the rings and returns to the InP chip. Of course otherconfigurations of this extended cavity in the Si are possible andcontemplated. The extended cavity may also be constructed from another,Si-compatible material, such as SiN. The Si chip is shown also includinga wavelength locker integrated onto the same chip for controlling thelaser wavelength. Also included in this configuration is a test gratingcoupler port such that the receiver and modulator may be tested on waferusing vertically coupled fibers.

While the methods, systems, and apparatus according to the presentdisclosure have been described with respect to particularimplementations and/or embodiments, those skilled in the art willrecognize that the disclosure is not so limited. Accordingly, the scopeof the disclosure should only be limited by the claims appended hereto.

1. An optical coherent transceiver comprising a monolithically integrated electrical and optical circuit having a substrate with a planar surface onto which is formed a polarization and phase-diversity coherent receiver and a polarization and phase-diversity modulator, and said optical coherent transceiver including only three optical Input/Output ports employing only three grating couplers, the first grating coupler optically connected to the polarization-diversity receiver, the second grating coupler optically connected to the receiver and the input of the polarization-diversity modulator, and the third grating coupler optically connected to the output of said modulator; wherein one of the grating couplers couples in to the transceiver a modulated signal, a second one of the grating couplers couples in to the transceiver a continuous-wave signal, and a third one of the grating couplers couples out a modulated signal.
 2. The optical transceiver according to claim 1 wherein the three grating couplers are connected to a fiber assembly including optical fiber having a bend radius less than 8 mm.
 3. The optical transceiver according to claim 1 wherein all three grating couplers are 2-D grating couplers.
 4. The optical transceiver according to claim 1 wherein one grating coupler is a 1-D grating coupler and the other two are 2-D grating couplers.
 5. The optical transceiver according to claim 1 wherein the substrate is a Si substrate.
 6. The optical transceiver according to claim 2 wherein the fiber assembly is vertical to within 2 degrees of the substrate surface.
 7. The optical transceiver according to claim 1 wherein TOPS are included in the combining paths of a grating coupler such that VOA functionality is provided.
 8. The optical transceiver according to claim 8 further comprising an external laser module that provides the cw signal.
 9. The optical transceiver according to claim 9 wherein said laser cw signal provides both transmitter and local oscillator function 